1. Field of the Invention
This invention relates to a method for making a unitary pattern assembly useful in investment casting and, more particularly, to a method for making a unitary pattern assembly useful in forming investment shell molds for the conventional and directional solidification of molten metals and alloys.
2. Description of the Prior Art
The present invention finds particular application in the production of gas turbine engine blades and like components. Turbine blades have been produced primarily by the method known as the "lost wax" process in which a wax pattern of the blade is provided and thereafter repeatedly dipped in a ceramic slurry, such as a zircon slurry, and dried until a shell of adequate thickness is formed thereon. The shelled pattern is then heated to a temperature that will melt the wax so that it flows out of the shell leaving behind a shell mold into which molten metal is poured and either conventionally or directionally solidified to produce the turbine blade. Obviously, the formation and maintenance of the wax pattern in the exact replica of the turbine blade is crucial in achieving a satisfactory casting, since any distortion in the wax pattern will be manifested in the shell mold formed therearound and in the subsequently cast turbine blade.
In the practice of the prior art, distortion of the wax pattern occurs most frequently during the manual assembly of the pattern assembly components. For example, it is common practice to form the pattern in a mold and manually remove the pattern therefrom. The pattern is joined to a wax coated metal support base, a sprue and runners are then joined to the pattern and a pour cup is thereafter joined to the sprue, the joining usually being done manually by wax welding. A handle is generally wax welded to the pour cup to provide a means for manipulating the pattern assembly during shell mold formation operations. If the assembly is structurally weak, a wax coated metal support plate may be wax welded to the pour cup and wax coated metal rods welded between the support base and support plate. This assembly procedure is one of the most critical operations in the entire investment casting process; if improperly done so as to distort the pattern, it can be a major cause of defective castings.
In efforts to overcome the inadequacies in the prior art practice, other workers have reported one-step and two-step injection molding processes for making a pattern assembly. In the one-step injection molding process, the pattern, runner and pour cup are formed as a one-piece assembly by the injection of molten wax into a suitable die in which a metal pour cup insert has been placed. After formation of the pattern assembly, a ceramic ring is wax welded to the pour cup to provide location and mechanical support means for subsequent processing operations. In the two-step injection molding process, the patterns are individually injected and then placed within an assembly die having sprue, runner and pour cup passages therein. A pour cup insert is placed in the die as in the one-step injection process. Molten wax is injected into the die to form a one-piece pattern assembly comprising the individual patterns connected to the sprue, runners and pour cup. A ceramic ring is then wax welded to the pour cup. The pattern assemblies produced by the disclosed processes are thereafter subjected to conventional shell mold formation operations.
Although an improvement over the prior art practice, the one-step and two-step injection molding processes suffer from several disadvantages. Both processes require that a metal pour cup insert be placed within the die prior to injection of the wax. Both processes also require that a ceramic ring be attached manually to the pour cup to provide location and support means for later operations. Also, neither process is amenable for use in the directional solidification of molten metals wherein the shell mold must have an open bottom to allow a chill plate to contact the molten metal, as taught in the Ver Snyder patent, U.S. Pat. No. 3,260,505. This is a serious disadvantage since modern gas turbine engines rely on directionally solidified turbine blades for improved performance characteristics. A disadvantage associated specifically with the one-step process is its limited capacity for producing a pattern assembly containing more than one or two patterns. Obviously, in the commercial production of turbine blades, it may be desirable to provide a pattern assembly having three, four or more patterns incorporated therein. Likewise, the two-step process is deficient in that the wax patterns are injection molded individually and must then be transported to and placed within the assembly die for attachment to sprue, runners and pour cup. Further, the joints between the pre-formed wax patterns and sprue are often characterized by objectionable roughness, such as ridges, which are a possible source of casting defects.